It is known, for example from US 2007/0256966, to treat stormwater through a bio-retention system. In such a system, stormwater, for example from the roof of a building, is directed to a container which contains a growing medium supporting plant growth. The stormwater percolates through the growing medium to the bottom of the container, and then flows to a mains drainage system. The stormwater thus provides a water supply for the growing plant, and the growing medium also filters the water to remove pollutants. Furthermore, the growing medium in the container serves as a retention device which regulates the flow of stormwater to the main drainage system, so as to reduce the load on the main drainage system during periods of heavy rainfall.
A problem with such systems is that the stormwater reaching the container may carry with it various pollutants which can interfere with the proper function of the growing medium. For example, if the container receives stormwater run-off from ground level surfaces such as roadways and pedestrian pavements, stormwater flow may include solid materials such as grit, leaves and litter and other pollutants associated with solids, such as heavy metals and other toxicants. It is preferable for such material not to be deposited on the growing medium.
Separators for separating such solid material from stormwater flows are known. For example, hydro-dynamic separators are low energy devices which operate on the principle of allowing a liquid containing suspended solid material to rotate in a cylindrical vessel so that the solid material falls under gravity to the base and there is swept to a central lower outlet by an inward sweeping effect caused by complex flow patterns in the device.
Such devices have been used in sewage treatment for separating hard grit from the incoming raw sewage, with the resultant de-gritted sewage then being passed to a conventional sewage treatment plant. They are also used as stormwater overflows, upstream of conventional sewage treatment works to ensure that gross contamination is separated from liquid waste discharged to watercourses during storm conditions when the sewage treatment works is unable to cope with the high flow. Separators of this type are described in GB2082941 and GB2158741, for example.